This invention relates to data communication systems, and, more particularly, to data communication systems for international modem calls.
With the advent of computers, high speed modems and computer networks, including the Internet, massive quantities and types of data have become readily accessible to computer users and are capable of being transferred between geographically remote computers.
Today, most telephone systems use digital transmission for both voice and modem calls, i.e., the transmission of data over telephone lines. For voice calls, the voice quality of a signal transmitted digitally at the receiving end of the call is superb to the human ear. However, for modem calls, the data received by the computer user at the receiving end of the call is often a distorted version of the originally transmitted data. The problem with modem calls that are transmitted between different countries is that the digital conversion standard used in some countries is different from that used in others. For example, the digital conversion standard used in Europe, known as A-law, converts the analog signal to different digital levels than the digital conversion standard used in the U.S., known as xcexc-law. The conversion directly between the two formats results in distortion, which while imperceptible in voice calls, wreaks havoc with data transmitted by modems, particularly high speed modems.
FIG. 1 illustrates a prior art data communication system that could be used to transmit international modem calls. Using an example of a data transmission from the United States to Europe, a modem signal is first generated by a local computer modem 10 in the United States. The local modem 10 converts the original computer data signal to an analog modulated data signal. Then, the analog modulated data signal is transmitted over an analog telephone line to a local central office 12. A converter 14, at the local central office 12, converts the analog modulated data signal into a xcexc-law digitized data signal. In particular, the converter 14 utilizes a xcexc-law table to convert the analog modulated data signal into a digital representation, comprised of a series of 8 bit digital values.
Next, the local central office 12 transmits the xcexc-law digitized data signal to a long distance carrier, such as ATandT, over a digital telephone line. The long distance carrier, at a local toll office 16 in the U.S., converts the xcexc-law digitized data signal to an A-law digitized data signal using a converter 18. Presently, the xcexc-law to A-law or A-law to xcexc-law conversion occurs in the toll office of the xcexc-law country. However, the xcexc-law digitized data signal could be transmitted over digital transmission lines from the local toll office 16 to a remote toll office 20 where the conversion occurs utilizing a converter 22. To make the direct xcexc-law to A-law conversion, the converter 18 utilizes a conversion table of all xcexc-law values and their corresponding A-law values. The converter 18, in essence, converts the 8-bit American digital representations into 8-bit European digital representations.
After converting the xcexc-law digitized data signal to the A-law digitized data signal, the local toll office 16 sends the A-law digitized data signal over digital transmission lines to a remote toll office 20 in Europe and then to a remote central office 24 where a converter 26 converts the A-law digitized data signal back to the analog modulated data signal. Finally, the recovered analog modulated data signal is transmitted over an analog telephone line to a remote computer where a remote computer modem 28 of the end user demodulates the analog modulated data signal to recover the original computer data.
The conversion distortion is introduced to the prior art data communication described above in two ways. First, because the original conversion from analog modulated data to xcexc-law digitized data requires a digital value to be selected, the digitally converted signal may not be reproduced perfectly even in native xcexc-law transmission. Second, when the conversion from xcexc-law to A-law digitized data occurs, there may not be an A-law value that exactly corresponds to the xcexc-law value, so another distortion can occur.
This type of distortion can be introduced into every sample, causing fluctuations that mimic, or obscure, the phase shifts that high speed modems often use for modulation. Slower speed modems (9600 bps and below) are less susceptible to the conversion distortion because there is more time to analyze the signal changes, and the distortion does not entirely block out the phase shifts or create artificial phase shifts. As a result, the modem signal delivered in Europe to the user, in the above example, has many additional changes beyond what the original modem created, and some of the original changes are now partially masked. This prevents high speed modulation from occurring effectively. Presently, the world is just about evenly split between A-law and xcexc-law countries, so the resulting digital conversion problem is not a uniquely American/European problem.
Another option for transmitting data internationally is for end users to subscribe to a global Internet service provider. However, subscribing to a global Internet provider is quite costly for end users. Furthermore, additional software is required to complete a dial-up connection from one personal computer to another for the purpose of file sharing, as the Internet service provider, and other data networks, require transmission of data in a particular protocol. There are also security issues associated with using a public data network like the Internet.
As a result, a need exists for a data communications system that can transmit international modem calls in a manner that eliminates the digital conversion distortion, from A-law to xcexc-law or xcexc-law to A-law.
To eliminate the distortion created by converting a modem signal from one digital format directly to another and to eliminate the modem speed penalty associated with such distortion, the present invention provides a data communication system that converts and demodulates a first digitized data signal to recover the original data signal and that re-modulates and converts the recovered original data signal to ultimately produce a second digitized data signal. The data communication system implements these functions with a digital signal processor, preferably controlled by a long distance carrier at either a local or remote toll office. Alternatively, the functions can be implemented by a separate bank of modems and converters or by two or more digital signal processors.
In accordance with an alternate embodiment of this invention, instead of implementing the conversion process of this invention at a single location at either the local or remote toll office as described above, the conversion process occurs at both ends of the call and utilizes a data network for internationally transmitting the original computer data. First, the conversion and demodulation of the first digitized data signal to recover the original data signal is implemented by a digital signal processor in an originating country. Then, the original data signal is transmitted over a data network from the originating country to a destination country. Thereafter, the re-modulation and conversion of the original data signal to a second digitized data signal is implemented by a digital signal processor in the destination country, where the second digitized data signal is then processed in a well-known manner.
In accordance with yet other aspects of the present invention, before converting and demodulating the first digitized data signal, the data communication system determines whether such conversion and demodulation needs to occur by identifying the call as a modem or voice call. If a modem call is detected, the data communication system proceeds with the conversion process of the present invention described above. However, if a voice call is detected, the data communication system processes the call in a well-known manner. Alternatively, the data communication system determines whether a special access code has been provided to implement the conversion process of this invention. If the access code has been provided, the data communication system implements the conversion process of this invention as described above. Otherwise, the data communication system processes the call in the conventional fashion.